This invention relates generally to cargo handling and, more particularly, to a method and apparatus for loading cargo onto or unloading cargo from a water vessel from or to a dock or quay.
Handling of cargo on water vessels is and has been accomplished by one of two usual methods, namely, a lift-on lift-off (Lo-Lo) system or a roll-on roll-off (Ro-Ro) system. As their names imply, the former system requires the use of derricks or cranes located on the vessel and/or the quay while the latter system employs ramps extending between the quay and the vessel on which cargo is rolled onto or off of the vessel.
Neither of these systems has proven to be entirely satisfactory however. Firstly, both systems require large investments in the form of quays and equipment. In the case of the lift-on lift-off system, the harbor cranes in conventional use, although movable to various locations on the quay, are limited with respect to the extent of their horizontal reach. This presents a problem where cargo must be stowed on a vessel at a location beyond the reach of the crane or, conversely, where cargo to be unloaded is located beyond the reach of the crane or not directly below it. The problem becomes particularly acute in the case of loading and unloading barges in shallow inland waterways where the vessel cannot be brought directly alongside the dock. In such cases, the crane operator is not always able to maintain visual contact with the cargo being carried by the crane and it is not unusual for such inconveniently positioned cargo to begin to sway upon being lifted by the crane. Such swaying involves a substantial risk of injury to personnel, cargo and equipment. In the case of the roll-on roll-off system, not only is it common for the water level in harbors to change during loading and/or unloading, but it is also evident that the draft of the vessel will change during loading or unloading of cargo. These conditions result in changes in the angular attitude of the ramps which extend between the vessel and quay on which the cargo is rolled and, frequently, the angle of the ramps become so steep as to prevent their use altogether.